1. Field of the Invention
The invention relates to improved catalysts for alkane dehydrogenations and to processes that include reactivating the partially spent catalysts.
2. Background of the Art
Conventional catalyst regeneration processes, that treat catalysts with reduced catalytic activity due, at least in part, to deposition of coke on catalyst surfaces, generally include removal of that coke. This is frequently accomplished by contacting such catalysts with air or another oxygen-containing gas under high temperature conditions. The temperature of such air or other gas may be, for example, greater than or equal to 450 degrees Celsius (° C.) for an ethanol dehydrogenation catalyst, or greater than or equal to 650° C. for a fluid catalyst cracking (FCC) catalyst. In some cases, however, conventional catalyst regeneration processes do not desirably restore catalytic activity of platinum containing supported gallium catalysts or other noble metal based (e.g., platinum-tin containing) dehydrogenation catalysts to a level equaling that of such catalysts when they are fresh. Thus, those who practice alkane dehydrogenation, especially propane dehydrogenation (PDH), understand that, as activity of a catalyst decreases, alkene production also decreases, eventually to a point where process economics dictate replacement of the deactivated catalyst with fresh catalyst. Since commercial viability depends upon optimization of economics, practitioners therefore desire means and/or methods to either more fully restore catalyst activity, or to otherwise delay the need to introduce fresh catalyst in these alkane dehydrogenations.
A typical regeneration of a noble metal based dehydrogenation catalyst involves many steps, frequently including coke combustion, drying and redispersion of the noble metal, and reduction. For example, U.S. Pat. No. 5,457,077 (Williamson, et al.) discloses a process for reconditioning platinum-containing catalyst particles that includes transferring the catalyst particles through both a combustion zone and a reconditioning zone. The reconditioning zone simultaneously effects drying of catalyst particles and redispersion of the platinum with a heated gas stream containing chlorine and oxygen.
U.S. Pat. No. 3,986,982 (Crowson, et al.) discloses chlorine regeneration of zeolite catalysts containing platinum group metals (e.g., platinum (Pt), palladium (Pd), rhodium (Rh), ruthenium (Ru), osmium (Os) and/or iridium (Ir)). This is done by (1) burning off deposits on the catalyst at no more than 500° C.; (2) treating the catalyst with inert gas, which is 0.5 volume percent (vol %) to 20 vol % oxygen and 5 parts by volume per million parts by volume (ppv) to 500 ppv chlorine at a temperature from 400° C. to 550° C.; (3) purging to remove residual oxygen and chlorine; and (4) reducing the catalyst in a stream of hydrogen gas at 200° C. to 600° C.
U.S. Pat. No. 2,773,014 (Snuggs, et al.) discloses hydrocarbon reforming with a platinum catalyst and a regeneration system for the catalyst. Regeneration involves bringing catalyst contained in a fixed reactor bed to an elevated temperature of about 850 degrees Fahrenheit (° F.) (˜454° C.) and burning off coke on the catalyst in the presence of a small amount of air. The catalyst is then rejuvenated by exposing it to (1) a circulating gas having an increased oxygen partial pressure of at least 0.4 atmosphere (39.2 kilopascals (kPa)) and (2) an increased bed temperature, e.g., the bed temperature is raised from 950° F. (˜510° C.) to 1200° F. (˜649° C.). Rejuvenation times depend upon the extent of catalyst deactivation, and range from 5 or 10 minutes, for slightly deactivated catalysts, to as long as 24 hours, for highly deactivated catalysts. Subsequent to rejuvenation the catalyst is purged of oxygen by introducing hydrogen to burn off the oxygen in the system.
British Patent (GB) 735,145 discloses a method for regenerating platinum and/or palladium catalysts that includes treating the catalysts at a temperature from 700° F. (˜371° C.) to 1600° F. (˜871° C.) with an oxygen-containing gas, where the oxygen partial pressure is from 5 pounds per square inch absolute (psia) (˜34.5 kPa) to 200 psia (˜1379 kPa). Oxygen associated with the catalyst is then removed by treating with hydrogen gas.
Despite these and other approaches to regeneration and rejuvenation of catalysts, those skilled in the art recognize that any means and/or method that reduces overall time and/or number of steps to accomplish such goal may potentially represent a significant cost savings in a given process. In view of this, it would be desirable in the art to discover means and/or processes to reduce such time or number of steps.